1. Field of the Invention
This invention relates to a self-vibrating brushless dc micro motor used for a portable wireless silent calling or paging device.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 5,036,239 (issued Jul. 30, 1991, one of the present applicants invented a flat coreless dc vibrator motor having neither output shaft nor external weight. This motor has a rotor which incorporates generally sector shaped non-overlapping flat armature coils arranged angularly partially about the shaft of the rotor, so that the center of mass of the rotor is off the rotational axis of the rotor.
As disclosed in U.S. Pat. No. 5,107,155 (issued Apr. 21, 1992), the same applicant invented another dc vibrator motor having neither output shaft nor external weight. This motor has a fixed shaft on which a cylindrical magnet is fixedly mounted, and an eccentrically-weighted rotor having a cylindrical coreless windings, enclosing the fixed magnet, is mounted on the fixed shaft. The rotor has either eccentric bearings or a combination of eccentric bearings and eccentric weights, so that the center of mass of the rotor is off its rotational axis. Although the motor has been favorably accepted in the market, the rotor of this motor requires additional production steps.
The same applicant also proposed a vibrator motor having a shaft which is rigidly fixed to a housing, and an eccentrically-weighted rotor mounted on the fixed shaft, as disclosed in the Japanese laid-open patent application, application No. 2-294482 (application date Oct. 31, 1990). In this motor, one of ordinarily three armature poles is deleted and the rotor has actually two armature poles angularly spaced at 120.degree. each other.
FIG. 10 shows a typical prior known dc vibrator motor. In FIG. 10, a cylindrical coreless dc vibrator motor M4 has an output shaft S on which an eccentric weight W, made of a tungsten alloy etc., is mounted. The motor vibrates as the shaft rotates because of the unbalanced distribution of the mass about the rotational axis.
However, some danger of mechanical interference pertains to this type of vibrator motor since the rotating eccentric weight is located outside the motor case. In addition, the alerting device requires an extra space in which the eccentric weight turns, and the high density material, such as tungsten alloy, for the eccentric weight is costly.
Furthermore, the sliding contacts between the brushes and the commutator segments conventionally used in such a cylindrical coreless dc motor have given rise to a life problem.